The present invention relates to a direct sequence code division multiple access (DS-CDMA) communication system and to a receiver for use in such a system.
In a DS-CDMA communication system a digital signal, for example digitised speech or data, is multiplied by a coding sequence comprising a pseudo-random sequence which spreads the energy in the modulating signal, which energy is transmitted as a spread spectrum signal. At the receiver, the antenna signal is multiplied by the same pseudo-random sequence which is synchronised to the spreading sequence in order to recover the modulating signal. Due to multipath effects which will cause intersymbol interference, Rake combining is used to overcome these effects and to produce a modulating signal which can be demodulated satisfactory.
In the case of a DS-CDMA communication system several different spread spectrum signals having the same or different chip rams and transmitted simultaneously at the same frequency by different users may be received at an antenna, each signal having been subject to different multipath effects, a method of equalisation which attempts to determine the channel impulse response and invert it is not adequate. Amongst the problems is what is known as the near-far effect due to signals from transmitters being received at a base station at different power levels. This effect is overcome by the base stations having fast power control algorithms.
In order for a receiver to be able to adapt itself to different conditions which may be found in practice, it must be able to cope with multiple bit rates which are required by a multi-media service provision, variations in the loading of the system, bit error degradation that other users' interference causes and power control failure caused, for example, by near-far interference under severe fading conditions.